1. Field of the Invention
The present invention relates to a method and an evaluation circuit for detecting the voltage in battery cells of a battery system that are preferably connected in series.
2. Description of the Prior Art
It is becoming apparent that in future, both in stationary applications such as wind power systems and in vehicles such as hybrid and electric vehicles, increasing use will be made of new battery systems that will have to meet very strict requirements with regard to reliability. The reason for these strict requirements is the fact that a failure of the battery can lead to a failure of the whole system, for example in an electric vehicle, a failure of the traction battery results in a so-called “stranded vehicle.” Another reason for these strict requirements is that a failure of the battery can also result in a safety problem; for example in wind power systems, batteries are used to adjust the rotor blades in the event of powerful winds so as to prevent the occurrence of impermissible operating states.
In order to achieve the required output and energy data with the battery system, usually individual battery cells are connected in series and sometimes also partially in parallel.
One problem with the use of many individual battery cells connected in series lies in the imperfect uniformity of the individual cells, which, particularly over their service life, results in unequal cell voltages if corresponding remedial action is not taken. Since overcharging or exhaustive discharging of individual cells results in irreversible damage to the battery—particularly in lithium-ion batteries, the voltages of the battery cells must be continuously monitored in order to permit initiation of countermeasures such as a cell balancing when necessary. As a rule, the monitoring of cell voltages is carried out using circuits that are either discreetly constructed or are used in an integrated fashion. FIG. 3 shows an example of such a circuit. In this case, a plurality of such dropping battery cell voltages, each from a respective battery cell 31a, 31b, . . . , 31f, are supplied one after another via a multiplexer 32a to an analog/digital converter 33a, which converts each analog value into a digital value. The cell voltages thus obtained are supplied via a connection 34a to a data bus that sends them to a microcontroller for further processing. Parallel to this, dropping battery cell voltages, each from a respective battery cell 31g . . . 31l, are supplied one after another via a multiplexer 32b to an analog/digital converter 33b, which converts each analog value into a digital value. The cell voltages thus obtained are supplied via a connection 34b to a data bus that sends them to the microcontroller for further processing. Parallel to this, dropping battery cell voltages, each from a respective battery cell 31m . . . 31r, are supplied one after another via a multiplexer 32c to an analog/digital converter 33c, which converts each analog value into a digital value. The cell voltages thus obtained are supplied via a connection 34c to a data bus that sends them to the microcontroller for further processing. Depending on the number of battery cells and the speed of the multiplexer components and analog/digital converter components used, it is possible as shown in FIG. 3 to divide the individual battery cells 31a . . . 31r of the battery system 31 into groups, each of which is associated with a respective multiplexer and an analog/digital converter. Alternatively, however, it is also possible to use one multiplexer and one analog/digital converter for all of the battery cells 31a . . . 31r of the battery system 31.
The disadvantages of this method include the time delay between the measurements, which increases with the number of cells per multiplexer, the resulting relatively low maximum possible scanning rate of the method, and the relatively high price of suitable a great circuits.